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Flow of energy and cycling of matter in ecosystems

Review your understanding of the movement of energy and matter in ecosystems with this free article aligned to NGSS standards.

Key terms

PhotosynthesisThe process by which plants, algae, and other photosynthetic organisms capture energy from sunlight to create organic molecules that can be used as food
Cellular respirationThe process by which organic molecules from food react chemically with other compounds, releasing energy that is used for essential life processes
ProducerAn organism that produces its own organic food molecules from inorganic sources, typically using energy from the sun
ConsumerAn organism that obtains organic molecules by consuming other organisms as food. Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers.
DecomposerAn organism that obtains energy by breaking down nonliving organic matter, such as discarded plant material, the remains of dead organisms, or animal waste
Food webA model that shows how matter and energy are transferred among producers, consumers, and decomposers in an ecosystem
Trophic levelAn organism’s position relative to the primary energy source (such as the sun) in a food chain
Ecological pyramidA model that represents the relative amount of matter and energy contained within each trophic level of an ecosystem

The movement of energy and matter in ecosystems

Energy flows through an ecosystem, while matter cycles within it. To understand why this is the case let’s take a closer look at how different life processes drive the movement of energy and matter in ecosystems.
Energy enters an ecosystem when producers carry out photosynthesis, capturing energy from the sun and storing it as chemical potential energy. During this process, matter from the environment (in the form of COA2 and HA2O) is taken in and rearranged into organic molecules (sugars). These organic molecules can power the producers’ life processes via cellular respiration (which releases COA2 and heat), or they can be stored as biomass.
Next, energy and matter move up the trophic levels of an ecosystem as producers are eaten by primary consumers, which are then eaten by secondary consumers, and so on. Some of the organic material eaten by consumers is used for cellular respiration (again, releasing COA2 and heat), some is stored as biomass, and the rest is excreted as waste.
Dead producers and consumers and their waste products provide matter and energy to decomposers. Decomposers transform matter back into inorganic forms that can be recycled within the ecosystem.
So, the energy that enters an ecosystem as sunlight eventually flows out of the ecosystem in the form of heat. In contrast, the matter in an ecosystem is continuously recycled as atoms are combined and recombined in different ways.

Energy and matter are conserved during ecosystem processes

As energy moves through an ecosystem, it changes form, but no new energy is created. Similarly, as matter cycles within an ecosystem, atoms are rearranged into various molecules, but no new matter is created. So, during all ecosystem processes, energy and matter are conserved.

Food webs model matter and energy transfer

A food web is a model of feeding relationships in an ecosystem. When an organism is eaten, the matter and energy stored in its tissues are transferred to the organism that eats it. The arrows in a food web represent this transfer.
For example, the arrow pointing from the mouse to the coyote in the food web below shows that matter and energy are transferred to coyotes when they eat mice.
A food web showing the feeding relationships in a grassland ecosystem.

Ecological pyramids model energy loss

Ecological pyramids show the relative amounts of matter or energy in different trophic levels of an ecosystem.
In most ecosystems, only about 10% of the total energy available at a given trophic level is transferred to the next level. The rest is used to power life processes, is discarded as waste, or is simply not consumed. An energy pyramid, such as the one below, illustrates this inefficiency by representing the energy available at each trophic level with a differently sized tier.
An energy pyramid showing the inefficient transfer of energy up trophic levels of an ecosystem.
A consequence of the inefficient energy transfer between trophic levels is that there tends to be fewer organisms at higher trophic levels in an ecosystem. The number of organisms at different trophic levels can be represented by a numbers pyramid, such as the one shown below.
A numbers pyramid showing the number of individuals per 0.1 hectare for each trophic level in a grassland ecosystem.

What else should I know about trophic levels and food webs?

  • Organisms can occupy more than one trophic level. Organisms are not limited to one trophic level. For example, omnivores (which eat plants and animals) can be classified as primary or secondary consumers.

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